A “mechanical finger” refers to an elongated, articulating, mechanical appendage. Like a human finger, a mechanical finger has one end joined to a structure that acts as a base and an opposite end that is not anchored or connected. A mechanical finger used for grasping typically has two or more rigid sections, and preferably at least three, connected end to end by articulating joints. Terminology used to describe the anatomy of a human finger is used to describe a mechanical finger. As in the human finger, each section of the finger is referred to as a “phalanx.” A finger extends from a base and is comprised of at least two, and preferably three, phalanges joined end to end by pivoting or articulating joints. A first articulating joint joins a proximal phalanx to a base, such as a palm of a hand. A second articulating joint joins the proximal phalanx to an intermediate or middle phalanx, and a third articulating joint joins the intermediate phalanx to a distal phalanx. The first joint is referred to as the metacarpophalangeal (MCP) joint, the second as the proximal interphalangeal (PIP) joint, and the third as the distal interphalengeal (DIP) joint.
In a mechanical finger, the phalanges are coupled to one or more motors to cause flexion and extension of the finger. When using a kinematic mechanism for coupling a single motor to the phalanges, the position of the actuator fully determines the position of the joints, but the torque at each joint is unknown. With a differential mechanism, the torque at the actuator determines the torque at each of the driven joints, but neither the velocity nor the position of the individual joints are specified by the actuator velocity or position alone. A kinematic mechanism produces consistent, predictable motion of the finger joints, but it does not allow the finger to curl around an object. Differential mechanisms allow curling and grasping, but often deviate from the desired motion due to forces at the fingertip, causing buckling, or due to friction in the joints, causing undesirable curling behavior when not conforming.